Multi-pitch and line calibration for mask and wafer CD-SEM system

ABSTRACT

The present invention relates to a system and method for calibrating a scanning electron microscope (SEM). The method comprises using a reference having multiple features of different dimensions and spatial interrelationships, wherein more than one feature dimension or spacing is measured using the SEM prior to measuring a workpiece. The dimensional and/or spatial measurements from the reference sample are correlated to obtain one or more calibration factors for the SEM. The calibration factor or factors may then be correlated with a workpiece SEM measurement to obtain a workpiece critical dimension (CD). A system is provided for calibrating a SEM including a reference with various measurable features of different dimensions and/or spacing. The system comprises an SEM to measure one or more reference sample feature dimensions and/or spacings and a processor or other device to correlate the measurement data to obtain one or more calibration factors.

TECHNICAL FIELD

The present invention generally relates to measurement systems andmethods and, more particularly, to a system and method for calibrating ascanning electron microscope.

BACKGROUND OF THE INVENTION

In the semiconductor industry there is a continuing trend toward higherdevice densities. To achieve these high densities there have been, andcontinue to be, efforts toward scaling down the device dimensions onsemiconductor wafers. In order to accomplish such a high device packingdensity, smaller features sizes are required. This may include the widthand spacing of interconnecting lines and the surface geometry such asthe corners and edges of various features.

The requirement of small features with close spacing between adjacentfeatures requires high resolution photo lithographic processes as wellas high resolution inspection and measurement systems and/or methods. Ingeneral, lithography refers to processes for pattern transfer betweenvarious media. It is a technique used for integrated circuit fabricationin which, for example, a silicon wafer is coated uniformly with aradiation-sensitive film (e.g., a photoresist), and an exposing source(such as ultraviolet light, x-rays, or an electron beam) illuminatesselected areas of the film surface through an intervening mastertemplate (e.g., a mask or reticle) to generate a particular pattern. Theexposed pattern on the photoresist film is then developed with a solventcalled a developer which makes the exposed pattern either soluble orinsoluble depending on the type of photoresist (i.e., positive ornegative resist). The soluble portions of the resist are then removed,thus leaving a photoresist mask corresponding to the desired pattern onthe silicon wafer for further processing.

In order to control quality in the design and manufacture of highdensity semiconductor devices, it is necessary to measure criticaldimensions (CDs) associated therewith. Semiconductor device featureshaving CDs of interest include, for example, the width of a patternedline, the distance between two lines or devices, and the size of acontact. CDs related to these and other features may be monitored duringproduction and development in order to maintain proper deviceperformance. As device density increases and device sizes decrease, theability to carry out quick, inexpensive, reliable, accurate,high-resolution, non-destructive measurements of CDs in thesemiconductor industry is crucial. The ability to accurately measureparticular features of a semiconductor workpiece allows for adjustmentof manufacturing processes and design modifications in order to producebetter products, reduce defects, etc.

CDs are usually measured during or after lithography. Various operationsperformed during the lithography process may affect the criticaldimensions of a semiconductor device. For example, variations in thethickness of the applied photoresist, lamp intensity during the exposureprocess, and developer concentration all result in variations ofsemiconductor line widths. In addition, line width variations may occurwhenever a line is in the vicinity of a step (a sudden increase intopography). Such topography-related line width variations may be causedby various factors including differences in the energy transferred tothe photoresist at different photoresist thicknesses, light scatteringat the edges of the steps, and standing wave effects. Moreover, issuesin connection with mask surface flatness, edge roughness of resistlines, charging of quartz material, chrome etch roughness, and phaseshifts also factor into critical dimensions. Since these factors cangreatly affect CDs, fast and reliable monitoring of semiconductor devicefeatures is important in order to guarantee acceptable deviceperformance.

Different technologies are currently available to measure CDs associatedwith semiconductor devices. These include: optical microscopy, stylusprofilometry, atomic force microscopy, scanning tunneling microscopy,and scanning electron microscopy. Scanning electron microscopes (SEMs)are commonly used for inspection and metrology in semiconductormanufacturing. The short wavelengths of scanning electron microscopeshave several advantages over conventionally used optical microscopes.For example, scanning electron microscopes may achieve resolutions fromabout 100 to 200 Angstroms, while the resolution of optical microscopesis typically about 2,500 Angstroms. In addition, scanning electronmicroscopes provide depths of field several orders of magnitude greaterthan optical microscopes.

In a typical SEM wafer inspection system, a focused electron beam isscanned from point to point on a specimen surface in a rectangularraster pattern. Accelerating voltage, beam current and spot diameter maybe optimized according to specific applications and specimencompositions. As the scanning electron beam contacts the surface of aspecimen, secondary electrons are emitted from the specimen surface.Semiconductor inspection, analysis and metrology is performed bydetecting these secondary electrons. A point by point visualrepresentation of the specimen may be obtained on a CRT screen or otherdisplay device as the electron beam controllably scans a specimen.

Scanning electron microscopes (SEMs) operate by creating a beam ofelectrons accelerated to energies up to several thousand electron volts.The electron beam is focused to a small diameter and scanned across a CDor feature of interest in the scanned specimen. When the electron beamstrikes the surface of the specimen, low energy secondary electrons(SEs) are emitted. The yield of secondary electrons depends on variousfactors including the work function of the material, the topography ofthe sample, the curvature of the surface, and the like. These electronscan be employed to distinguish between different materials on a specimensurface since different materials may have significantly different workfunctions.

Topographic features also affect the yield of electrons. Consequently,changes in height along a specimen surface may be measured using an SEM.Electron current resulting from the surface-emitted secondary electronsis detected and used to control the intensity of pixels on a monitor orother display device connected to the SEM. An image of the specimen maybe created by synchronously scanning the electron beam and the displaydevice.

Although SEMs can achieve resolution in the range of angstroms,calibration is difficult. For example, the magnification of an SEM maybe calibrated by placing a sample of known dimensions, such as a chip orwafer having a conductor line of known width, in the instrument andmeasuring the line width of the sample. The magnification of the SEM isdetermined by dividing the SEM measurement of the image of the sample bythe known dimension of the sample. The magnification calibrationinformation may then be used to construct a calibration curve, or theSEM's magnification controls may be trimmed accordingly.

Calibration according to these prior methods requires samples of knowndimensions. Even if the actual dimensions of a sample are known,however, they may change. In particular, repeated usage of a singlereference sample as a calibration standard results in degradation of thereference sample. Charge buildup on a reference sample caused byrepeated measurement in an SEM affects the secondary electron emission.Contaminant deposition or buildup also has deleterious effects onmeasurement of a calibration standard reference sample over time.Conventional SEM calibration methods and systems do not account fordegradation in reference features. Thus, repeated use of the samereference sample may lead to SEM calibration error over time, as thereference sample feature size changes.

The measurement of a calibration standard reference sample typicallyinvolves determining where an edge of the sample is. At the sub-micronrange, an edge of a sample may be a complex waveform, as opposed to aflat line. Therefore, in measuring the sample, assumptions must be madeas to edge location, which lead to errors. Where the calibrationinvolves determining the length (or width) of a sample, two edges mustbe located, and thus the edge determination errors are doubled. Further,sample dimensions may vary as a function of temperature. The SEMelectron beam may thus cause expansion of a reference sample afterrepeated use.

In order to reduce edge determination error, SEM calibration has alsobeen done using a sample having a series of equally spaced lines. Such asample could be a diffraction grating having a plurality of alignedparallel grooves. The SEM may be used to measure the pitch of the lines.While this method reduces some of the edge quantification errorsassociated with other SEM calibration methods, higher accuracycalibration methods are needed for SEMs used for measuring high densitysemiconductor devices.

Conventional SEM calibration methods and systems do not account fordegradation of a calibration standard reference sample over time. Forexample, where a line width feature on a reference sample has a knownwidth, repeated scanning of the feature by an SEM results in chargebuildup. This reduces or hampers the ability of an SEM to obtainaccurate measurements of the line width in the future. Corrosiondeposition on a reference sample feature also prevents or hampersaccurate readings. Mask references are particularly susceptible tocarbon contamination. Because conventional SEM calibration methods andsystems rely upon accurate SEM readings of a known reference featuredimension, inaccurate SEM readings of a calibration standard referencefeature cause errors in measurements of workpiece features performedwith the SEM.

SUMMARY OF THE INVENTION

The present invention provides a method and system for calibrating ascanning electron microscope, which minimizes or reduces thedisadvantages associated with conventional methods and systems. Inaccordance with one aspect of the present invention, there is provided amethod and system for calibrating an SEM using a reference havingmultiple features of different dimensions and/or spatialinterrelationships, wherein more than one feature dimension or spacingis measured using the SEM prior to measuring a workpiece. Thedimensional and/or spatial measurements from the reference sample arecorrelated to obtain one or more calibration factors for the SEM. Thecalibration factor or factors may then be correlated with a workpieceSEM measurement to obtain a workpiece critical dimension (CD). Themethod and system eliminate or minimize the effects of reference featuredimension variations, allowing such deviations to be detected andaccounted for in the calibration of a SEM. In this regard, thecorrelation of the reference feature dimensions may comprise one or moreof computing the slope of a curve, computing a zero offset, computing acalibration coefficient, curve fitting, stochastics, neural networks,artificial intelligence, data fusion techniques, and/or trending,according to another aspect of the invention.

According to another aspect of the present invention, one or morecorrelation curves may be generated from the reference sample SEMmeasurements for analysis in calibrating the SEM. The curves maycomprise measured line width or pitch versus actual line width or pitch,or even actual or measured width versus actual or measured pitch. Inthis way, the correlation may utilize curve fitting, stochastics, neuralnetworks, artificial intelligence, data fusion techniques, trending, andthe like, to account for variations in reference sample featuredimensions in determining one or more calibration factors for the SEM.

In accordance with another aspect of the present invention, there isprovided a method for calibrating a scanning electron microscope,comprising providing a reference sample having a first line with a firstline width, and a second line with a second line width, and measuringthe first and second line widths using the scanning electron microscope.The method further comprises correlating the first line widthmeasurement with the second line width measurement to obtain at leastone calibration factor.

In accordance with yet another aspect of the present invention, there isprovided a method for calibrating a scanning electron microscope,comprising providing a reference sample having a first line set withgenerally parallel lines of a first pitch, and a second line set withgenerally parallel lines of a second pitch, measuring the first pitchusing the scanning electron microscope, and measuring the second pitchusing the scanning electron microscope. The first pitch measurement andthe second pitch measurement are then correlated to obtain at least onecalibration factor for the SEM.

In accordance with still another aspect of the invention, there isprovided a method for calibrating a scanning electron microscope,comprising providing a reference sample having a first line set withgenerally parallel lines of a first line width and a first pitch, and asecond line set with generally parallel lines of a second line width anda second pitch, measuring at least one of the first line width and thefirst pitch using the scanning electron microscope, and measuring atleast one of the second line width and the second pitch using thescanning electron microscope. The method further comprises correlatingat least one of the first line width measurement and the first pitchmeasurement with at least one of the second line width measurement andthe second pitch measurement to obtain at least one calibration factor.The correlation may comprise one or more of computing the slope of acurve, computing a zero offset, computing a calibration coefficient,curve fitting, stochastics, neural networks, artificial intelligence,data fusion techniques, and trending, in order to calibrate the SEM.

Another aspect of the invention provides for measuring a workpiecefeature using the scanning electron microscope to obtain a workpiecefeature measurement, and correlating the workpiece feature measurementwith at least one calibration factor to obtain a workpiece feature CD.In this way, the calibration factor or factors may be employed to adjustworkpiece feature measurements in order to determine or obtain accurateworkpiece feature CDs.

According to yet another aspect of the present invention, a system isprovided for calibrating an SEM. The system may be utilized to implementthe above methods according to the invention. The system may comprise areference sample having a first line set with generally parallel linesof a first line width and a first pitch, and a second line set withgenerally parallel lines of a second line width and a second pitch, ascanning electron microscope adapted to measure at least one of thefirst line width and the first pitch, and at least one of the secondline width and the second pitch, and a processor or other device adaptedto correlate at least one of the first line width measurement and thefirst pitch measurement with at least one of the second line widthmeasurement and the second pitch measurement to obtain at least onecalibration factor. The system provides for reduction or elimination ofcalibration errors associated with changing reference sample featuressuch as line width, which were not accounted for in convention SEMcalibration.

In accordance with yet another aspect of the invention, there isprovided a system for calibrating a scanning electron microscope,comprising a reference sample having a first line set with generallyparallel lines of a first line width and a first pitch, and a secondline set with generally parallel lines of a second line width and asecond pitch, a device for measuring at least one of the first linewidth and the first pitch, a device for measuring at least one of thesecond line width and the second pitch, and a device for correlating atleast one of the first line width measurement and the first pitchmeasurement with at least one of the second line width measurement andthe second pitch measurement to obtain at least one calibration factor.The calibration system accounts for degradation in a reference sampleassociated with repeated usage in a SEM, and further allows trendinganalysis of the reference sample degradation. In this way, a user mayidentify degradation in a reference sample feature, and take appropriateaction without a corresponding degradation in SEM workpiece measurementaccuracy.

To the accomplishment of the foregoing and related ends, the invention,then, comprises the features hereinafter fully described andparticularly pointed out in the claims. The following description andthe annexed drawings set forth in detail certain illustrative examplesof the invention. These examples are indicative, however, of but a fewof the various ways in which the principles of the invention may beemployed. Other objects, advantages and novel features of the inventionwill become apparent from the following detailed description of theinvention when considered in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating a method for calibrating ascanning electron microscope in accordance with the present invention;

FIG. 2 is a schematic diagram illustrating a conventional scanningelectron microscope;

FIG. 3 is a schematic diagram illustrating a conventional system forcalibrating a scanning electron microscope;

FIG. 4 is a schematic diagram illustrating a system and method forcalibrating a scanning electron microscope in accordance with theinvention;

FIG. 5 is a schematic diagram illustrating the system and method forcalibrating a scanning electron microscope of FIG. 4;

FIG. 6 is a plan view of an exemplary reference sample which may be usedin the methods and systems of the present invention;

FIG. 7 is a plan view of another exemplary reference sample which may beused in the methods and systems of the present invention;

FIG. 8 is a plan view of still another exemplary reference sample whichmay be used in the methods and systems of the present invention;

FIG. 9 is a plan view of yet another exemplary reference sample whichmay be used in the methods and systems of the present invention;

FIG. 10 is a plan view of yet another exemplary reference sample whichmay be used in the methods and systems of the present invention;

FIG. 11a is an exemplary graph illustrating a correlation of width andpitch which may be used in the methods and systems of the presentinvention;

FIG. 11b is another exemplary graph illustrating a correlation of widthand pitch which may be used in the methods and systems of the presentinvention;

FIG. 11c is another exemplary graph illustrating a correlation of widthand pitch which may be used in the methods and systems of the presentinvention;

FIG. 12a is an exemplary graph illustrating a correlation of measuredand actual pitch which may be used in the methods and systems of thepresent invention;

FIG. 12b is another exemplary graph illustrating a correlation ofmeasured and actual pitch which may be used in the methods and systemsof the present invention;

FIG. 12c is another exemplary graph illustrating a correlation ofmeasured and actual pitch which may be used in the methods and systemsof the present invention;

FIG. 13a is an exemplary graph illustrating a correlation of measuredand actual width which may be used in the methods and systems of thepresent invention;

FIG. 13b is another exemplary graph illustrating a correlation ofmeasured and actual width which may be used in the methods and systemsof the present invention;

FIG. 13c is another exemplary graph illustrating a correlation ofmeasured and actual width which may be used in the methods and systemsof the present invention;

FIG. 14a is another exemplary graph illustrating a correlation ofmeasured and actual width which may be used in the methods and systemsof the present invention;

FIG. 14b is another exemplary graph illustrating a correlation ofmeasured and actual width which may be used in the methods and systemsof the present invention;

FIG. 14c is another exemplary graph illustrating a correlation ofmeasured and actual width which may be used in the methods and systemsof the present invention; and

FIG. 15 is another exemplary graph illustrating a correlation ofmeasured and actual width which may be used in the methods and systemsof the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will now be described with reference to thedrawings, wherein like reference numerals are used to refer to likeelements throughout. The methods and systems of the invention reduce thecalibration errors in scanning electron microscopes by measuringmultiple feature dimensions and/or multiple line set pitches, andcorrelating the measurements to obtain one or more calibration factors.This provides for reduction or elimination of errors due to referencefeature dimension degradation, as described in more detail infra. FIG. 1illustrates a method 2 for calibrating a scanning electron microscope. Areference sample is provided having first and second line sets at step4. The first and second line sets have a line width and pitch associatedtherewith, at least one of which are measured at steps 6 and 8,respectively, with an SEM. The line set measurements are then correlatedat step 10 whereby at least one calibration factor is obtained. Thecorrelation may comprise one or more of computing the slope of a curve,computing a zero offset, computing a calibration coefficient, curvefitting, stochastics, neural networks, artificial intelligence, datafusion techniques, and trending, in order to calibrate the SEM. By thiscalibration method, degradation in reference feature sizes may beaccounted for, and the degraded reference features may be identified bya user. Thus, system accuracy may be preserved, even where one or morereference sample features have degraded.

Once the calibration factor or factors have been obtained via referencesample SEM measurements, a feature of interest on a workpiece may bemeasured at step 12 using the SEM, and a workpiece feature CD isobtained at step 14 by correlating the workpiece SEM measurement with atleast one calibration factor. In a typical application, the calibrationmethod may be employed periodically or regularly, whereby referencesample degradation trends may be both monitored and accounted for in thecalibration correlation. Although a reference sample feature maydegrade, as for example, through repeated measurement in a SEM, themethod of the present invention allows accurate calibration of the SEMinstrument through the use of one or more calibration algorithms, as forexample, computing the slope of a curve, computing a zero offset,computing a calibration coefficient, curve fitting, stochastics, neuralnetworks, artificial intelligence, data fusion techniques, and trending.

The method of FIG. 1 reduces or eliminates the effects of referencesample degradation on the CD measurement of the workpiece, since thevarious line width and pitch measurements of the reference sample may beused to correlate the SEM reference sample measurement. In particular,charge accumulation and contamination deposition associated withrepeated usage of a reference sample in a SEM system, may be accountedfor because, while a particular line width on the sample may change, thepitch of a line set generally will not. Furthermore, a user may performtrending analysis to determine the extent of a reference sample'sdegradation, and calibrate out the effects associated therewith throughthe correlation of the various line width and pitch measurements. Inaddition to those mentioned above, the effects associated with carboncontamination and charge accumulation, as well as temperature effects,may be taken into account in the correlation.

The aspects of the present invention will be further illustratedhereinafter, in comparison with convention SEM calibration techniquesand systems, which are discussed to provide context for the invention.In particular, FIG. 2 illustrates a CD-SEM system 20 including a chamber24 for housing a wafer 26. In order to measure the wafer 26, an electronbeam 28 is directed from an electromagnetic lens 30 toward the wafer 26.The electron beam 28 is created from high voltage supplied by a powersupply 32 associated with a beam generating system 34 which includes anemission element 34 a. Various directing, focusing, and scanningelements (not shown) in the beam generating system 34 guide the electronbeam 28 from the emission element 34 a to the electromagnetic lens 30.The electron beam particles may be accelerated to energies from about500 eV to 40 Kev. When the electron beam 28 strikes the surface of thewafer 26, electrons and x-rays are emitted which are detected by adetector 36 and are provided to a detection system 38. The detectionsystem 38 provides signals to a processing system 44 for performingconventional critical dimension measurements, for example, to determinethe width of a line or other feature of interest on the wafer 26.

Electrons which are emitted from the surface of the wafer 26 which aremost useful for critical dimension imaging are known as secondaryelectrons and provide a substantial amount of the signal currentreceived by the detector 36. A dimension image may also be directed to adisplay 40 by the processing system 44. The processing system 44, inaddition to processing data received by the detection system 38,synchronizes the scanning of the display 40 with electron beam scanningof the wafer 26 to provide the image. Contrast of the displayed image isrelated to variations in the flux of electrons arriving at the detector36 and is related to the yield of emitted electrons from the surface ofthe wafer 26 to the incident electrons from the electron beam 28.

The detection system 38 receives the electron emissions from the surfaceof the wafer 26 via the detector 36 and may digitize the information forthe processing system 44. In addition, the detection system 38 may alsoprovide filtering or other signal processing of the received signal. Theprocessing system 44 provides dimension information to the display 40and/or stores information in a memory 46.

A processor (not shown) is included in the processing system 44 forcontrolling the beam generating system 34, providing critical dimensionmeasurements, and for performing signal analysis. It is to beappreciated that a plurality of processors and/or processing systems maybe included as part of and/or external to the CD-SEM system 20. Theprocessor in the processing system 44 is programmed to control andoperate the various components within the CD-SEM system 20 in order tocarry out the various functions associated with the measurement of thewafer 26.

A memory 46 is also included in the system 20. The memory 46 isoperatively coupled to the processing system 44 and serves to storeprogram code executed by the processor for carrying out operatingfunctions of the system 20. The memory 46 also serves as a storagemedium for temporarily storing information such as calibration data,critical dimension data, and other data. The power supply 32 alsoprovides operating power to the CD-SEM system 20 along with providing ahigh voltage to the beam generating system 34.

Referring now to FIG. 3, a conventional SEM calibration is shown,wherein a reference sample 50 having a reference feature 52 is locatedon a stage 54 and measured by the SEM system 20. The electron beam 28 isdirected onto the feature 52, and the detector 36 senses the secondaryelectrons 56, in order to obtain an optical or SEM measurement of thefeature 52 as the stage 54 is displaced relative to the lens 30 in oneor more directions perpendicular to the electron beam 28.

Where a physical characteristic of the feature 52, such as for example,the width of a conductor line is of interest, the sample 50 may bescanned via movement of the stage 54, whereby a change in the secondaryelectrons 56 can be determined and a distance (e.g., line width)calculated. Where the line width of the reference feature 52 is known,the reference feature measurement may be used to calibrate the SEM priorto its use in measuring features of a workpiece, such as a semiconductorwafer 26. However, where the dimensions or other characteristics of thereference sample feature 52 are not preceisely know, or change, due tocontamination, charge buildup, heat, or other causes, the conventionalcalibration method of FIG. 3 will not prevent errors in measuringworkpiece feature CDs.

Referring now to FIG. 4, a system and method for calibrating a scanningelectron microscope are illustrated and described in accordance with thepresent invention. The system 100 comprises a beam generator 134 havingan emission element 134 a which provides an electron beam 128 through alens 130 to a reference sample 150 and/or a workpiece 126, which aremounted on a stage 154 within a chamber 124, as will be discussed ingreater detail infra. Secondary electrons 156 are sensed by a detector136 which is operatively coupled to a detection system 138. A processingsystem 144 is provided which receives information from the detectingsystem 138. The processing system 144 provides critical dimensioninformation to a display 140 and/or stores information in a memory 146.A processor (not shown) is included in the processing system 144 forcontrolling the beam generating system 134, providing critical dimensionmeasurements, and for performing signal analysis. A plurality ofprocessors and/or processing systems may be included as part of and/orexternal to the CD-SEM system 100 for performing signal analysis andother operations associated with performing the measurement andcalibration methods according to the invention.

The processor in the processing system 144 is programmed to control andoperate the various components within the CD-SEM system 100 in order tocarry out the various functions described herein. The processor may beany of a plurality of processors, for example, such as the AMD Athlon,K6 or other type architecture processors. The memory 146 is operativelycoupled to the processing system 144 and serves to store program codeexecuted by the processor for carrying out operating functions of thesystem 100 as described herein. The memory 146 also serves as a storagemedium for temporarily storing information such as calibration data,critical dimension data, statistical data, trending information, andother data which may be employed in carrying out the present invention.The power supply 132 also provides operating power to the CD-SEM system100 along with providing a high voltage to the beam generating system134. Any suitable power supply (e.g., linear, switching) may be employedto carry out the present invention. In addition, the stage 154 isadapted to move horizontally with respect to the vertical electron beam128 to effect a scanning of the workpiece 126 and/or the referencesample 150 by the beam 128.

In addition to the programs and instructions for carrying out SEMmeasurements, the processing system 144 is adapted to receive andanalyze data (not shown) relating to pitch and/or line widthmeasurements of various features 162 on the reference sample 150. Inthis regard, the processing system 144 and/or the memory 146 may includeprograms and instructions for performing various mathematicalalgorithms, such as for example, computation of one or more calibrationor scaling factors, stochastics, neural networks, artificialintelligence, data fusion techniques, and the like. These algorithms maybe advantageously used to correlate reference feature measurement datarelating to feature size and/or spacing (e.g., line width and pitch) inorder to calibrate the SEM system 100 as described in greater detailinfra. In particular, the processing system 144 may be used to correlatereference pitch and/or width measurements of reference features 162 inorder to obtain or determine one or more calibration factors, andfurther to correlate a workpiece feature SEM measurement with thecalibration factor or factors to obtain or determine a workpiece featureCD, in accordance with one aspect of the present invention. Thecorrelation of reference feature dimension and spacing information(e.g., line width and pitch) reduces or eliminates prior calibrationerrors associated with charge buildup, corrosion, and other degradationof a reference sample resulting from repeated usage in a SEM,temperature change, and the like.

Reference sample feature dimensions and/or spacing are thus measuredusing the system 100. With the stage 154 positioned as shown in FIG. 4and a beam 128 generated by the beam generating system 134, the system100 may perform an SEM measurement of the feature 162 a. The stage maythen be moved horizontally in order to effect a scanning by the beam 128of the line features 162 a, 162 b and 162 c, which form a first lineset. The line features 162 a, 162 b, and 162 c are illustrated as havingapproximately equal line widths (not numerically designated), and pitch164. The stage 154 may then be moved horizontally to measure thefeatures 162 d and/or 162 e of the reference sample 150. The features162 d and 162 e are of approximately equal line width (not numericallydesignated), and form a second line set having a pitch 166. The systemmay measure the line widths of features 162 a, 162 b, 162 c, and thoseof features 162 d and 162 e, as well as the pitches 164 and/or 166 ofthe first and second line sets. The measurements may then be correlatedto obtain one or more calibration factors, as discussed in greaterdetail infra.

It will be appreciated that the reference feature measurements may be oftwo or more lines of different widths, or of two or more line sets ofdifferent pitches, whereby a correlation can be achieved by variousprocessing techniques. In addition, although the features 162 areillustrated as two sets of parallel lines, with the lines within eachset having generally equal line widths, other embodiments arecontemplated as within the scope of the present invention, such as, forexample, a line set having lines of various widths with equal pitch,multiple line sets having lines of generally equal width at varyingpitches, and features which are non-linear.

With the reference feature measurements thus obtained, the processingsystem 144 may then correlate some or all of the dimensional and spatialinformation (e.g., line width and pitch data) in order to obtain one ormore calibration factors for the system 100. This correlationadvantageously provides for calibration of the SEM measurement, and mayinclude compensation for variations in one or more reference featuresizes, as well as corrosion, charge accumulation, temperature, and otherdeleterious degradation of a reference sample feature 162.

As an example, where the electrical and optical reference featuremeasurements are taken periodically (e.g., daily, weekly, etc.),trending analysis can be performed by the processing system 144, wherebythe degradation of a reference sample feature 162 may be determined. Asan example, trending may show that the SEM measurement of the line widthof reference feature 162 a remains fairly constant over a period oftime, while the width of feature 162 c decreases (or increases) over thesame timer period. This may be used, for example, to determine that anew reference sample is needed, or that the data associated with feature162 c may be discounted in future calibrations.

In addition to trending analysis, the processing system 144 may furtherperform data fusion analysis, wherein the dimensional and/or spacingmeasurements are analyzed to determine other variables affecting systemperformance. In this context, data fusion is algorithmic processing ofmeasurement data or information to compensate for the inherentfragmentation of information because a particular phenomena may not beobserved directly using a single sensing element or measurement. Inother words, the data fusion architecture provides a suitable frameworkto facilitate condensing, combining, evaluating and interpreting theavailable optical and electrical measurement data or information in thecontext of the particular application, such as an SEM system 100.

It will be appreciated that the degradation of a reference samplefeature 162 may affect the dimensional and spacing measurements thereofdifferently, and that the correlation thereof provides for improvedcalibration capabilities within the present invention. For example, thewidth of a line 162 a in a first line set comprising line features 162a, 162 b, and 162 c, may change, while the pitch of the line set remainsconstant. In this regard, the use of pitch information in thecalibration correlation, may provide an indication that a particularline width has changed. Moreover, it will be recognized that manyalgorithms and correlation techniques are available to compensate forreference sample feature degradation, variation, and other errors in thesystem 100, for example, computing calibration scaling factors, curvefitting, stochastics, neural networks, artificial intelligence, datafusion techniques, mathematical prediction/correction techniques, andthe like.

Referring also to FIG. 5, once the calibration factor or factors havebeen obtained or determined, the stage 154 may positioned such that theworkpiece 126 is beneath the electron beam 128. Thereafter, the system100 may perform an SEM scan of one or more features of interest (notshown) on the workpiece 126, with workpiece feature measurement data orinformation being provided to the processing system 144 via thedetection system 138. The processing system 144 may then correlate theworkpiece feature measurement data with one or more calibration factorsin order to obtain or determine a workpiece feature CD. As with thecorrelation of the reference sample dimensional and/or spacingmeasurements discussed supra, the correlation of the calibration factoror factors with the SEM workpiece feature measurement by the processingsystem 144 may comprise, for example, curve fitting, stochastics, neuralnetworks, artificial intelligence, data fusion techniques, mathematicalprediction/correction techniques, and the like.

It will be appreciated that the workpiece feature measurement may beperformed prior to the reference sample measurements, and further thatthe reference feature measurements (dimensional and spatial) may beperformed in any order or simultaneously. Furthermore, the measurementsneed not be performed contemporaneously, since the correlationalgorithms and trending analysis may account for the time the variousmeasurements are made. In this way, a workpiece CD may be obtained for aworkpiece measured, for example, a week prior to the calibrationreference feature measurements used to correlate the workpiecemeasurements, etc.

Referring now to FIG. 6, an exemplary reference sample 200 isillustrated, having multiple parallel linear features 202 located so asto form line sets 204, 206, 208, and 210. In the reference sample 200,the widths of the line features 202 are generally equal, and the pitches212, 214, 216, and 218, respectively, of the line sets 204, 206, 208,and 210 are different. An SEM, such as that of the system 100 of FIGS. 4and 5, may scan along a scan line 220 (or other paths generally parallelthereto) one or more times in order to measure the widths of the linefeatures 202, and/or the pitches 212, 214, 216, and/or 218 of the linesets 204, 206, 208, and/or 210, respectively. The line width and/orpitch measurements may then be correlated by the processing system 144of system 100 to obtain one or more calibration factors. Thesecalibration factors, in turn, may be employed to provide workpiecefeature CDs based on correlation of a workpiece feature measurement andthe calibration factor or factors.

Referring now to FIG. 7, another exemplary reference sample 230 isillustrated having line sets 232, 234, 236, and 238, wherein the linefeatures 242, 244, 246, and 248, respectively therein, are of generallyequal width. The line sets 232, 234, 236, and 238 have pitches 252, 254,256, and 258 associated therewith, respectively. The reference sample230 allows SEM measurement of several different line widths 262, 264,266, and/or 268, as well as several different pitches 252, 254, 256,and/or 258, along a scan line 260, which may be correlated to obtaincalibration factors in accordance with the invention. Referring also toFIG. 8, another reference sample 270 is illustrated having a single lineset 272 comprising six line features 274, 276, 278, 280, 282, and 284with line widths 286, 288, 290, 292, 294, and 296, respectively. Theline set 272 comprises a single pitch 298. This allows an SEM scan alongscan line 299 to measure several different line widths and a singlepitch.

Several different combinations of line width and pitch patterns may becombined in one reference sample, as are illustrated in FIGS. 9 and 10.In particular, FIG. 9 illustrates one exemplary reference sample 300wherein line features 302 are provided in three rows 304, 306, and 308.The feature lines 302 are grouped into line sets 310, 312, 314, and 316within each row, having pitches 320, 322, 324, and 326, respectively.The provision of multiple rows 304, 306, and 308 of line sets 310, 312,314, and 316 in the reference sample 300 allows more data to be measuredfor correlation in determining the calibration factors according to theinvention. Thus, the SEM system 100 may scan the sample 300 along scanlines 330, 332, and 334 each time the system 100 is calibrated. Asanother example, the system 100 may scan only row 304 along scan line330. Where it is found via the reference feature measurementcorrelation, that certain reference feature lines 302 in row 304, havechanged, the other rows 306 and/or 308 may be utilized by scanning alonglines 332 and/or 334 without requiring a new reference sample.

Referring also to FIG. 10, another exemplary reference sample 350 isillustrated, in which multiple line features (not designatednumerically) are located in three rows 352, 354, and 356. In thisexample, the reference sample 350 provides the capability of measuringseveral line sets 360, 362, 364, and 366 of lines of generally equalwidth (not numerically designated) in the row 352, with the pitch (notnumerically designated) associated with each line set 360, 362, 364, and366 being different. In this regard, the SEM system 100 may measure theline features of row 352 along a scan line 368. Row 354 may be used formeasurement of several line sets 370, 372, 374, and 376 along a scanline 378, wherein different line widths and pitches (not numericallydesignated) are provided. Row 356 may be scanned along a scan line 380,whereby a reference measurement may be made of the line features (notnumerically designated) arranged with varying widths and a single pitch.The reference samples shown in FIGS. 4-10 illustrate severaladvantageous configurations of linear features which may be used inimplementing the systems and methods of the present invention. Manyother reference samples are possible having different configurations offeatures measurable by an SEM, and are considered as falling within thescope of the invention. In this regard, the features need not be linear,nor need they be arranged in rows or columns.

Referring now to FIGS. 11a, 11 b and 11 c, measured reference featuredata may be correlated using curve fitting techniques, in order toobtain one or more calibration factors for the SEM system 100. Severalpossible correlations are hereinafter described for illustration ofvarious aspects of the invention. However, it will be appreciated thatmany different correlation methodologies fall within the scope of thepresent invention, and that those which are discussed hereinafter areillustrative of the invention, and not a limitation thereon. Thereference sample feature dimensional and spatial values may be initiallyknown.

Referring also to FIG. 10, and particularly to row 354, an SEM scanalong line 378 allows measurement of line sets 370, 372, 374, and/or376, whose line widths and pitches (not numerically designated) mayinitially be known. The line sets 370, 372, 374, and 376 each includeline widths approximately one half the pitch, wherein the line widthapproximates the spacing between the line features. In the graph 400 ofFIG. 11a, a line 402 represents the theoretical correlation betweenfeature line width on the Y axis and line set pitch on the X axis.Because of the arrangement of the line sets in row 354 of sample 350,the line 402 is straight, having a constant slope 404 and no Y axisoffset. Upon taking SEM line width and pitch measurements along scanline 378, a measurement correlation curve 406a may be determined usingcurve fitting or other mathematical techniques.

In FIG. 11a, data points 410 a, 412 a, 414 a, and 416 a correspond towidth and pitch measurements of line sets 370, 372, 374, and 376,respectively. In this example, it is seen that the measurementcorrelation curve 406a is above and generally parallel with thetheoretical line 402, thus the measurement curve slope 418 a will be thesame as the slope 404 of curve 402. An exemplary correlation, such asthat of FIG. 11a, may thus indicate that the line width measurementcapability of the SEM is in need of adjustment (e.g., span, gain, ormagnification adjustment) to calibrate out the effects of the Y axisoffset 420 between the curves 402 and 406 a. This situation may alsoindicate that the line widths have increased, for example, due to chargebuildup, temperature effects, and the like. The correlation may then beused to calculate, determine, or otherwise obtain one or morecalibration factors via computing a zero offset, computing a calibrationcoefficient, curve fitting, stochastics, neural networks, artificialintelligence, data fusion techniques, trending, or other techniques. Forexample, the theoretical slope 404, the measurement curve slope 418 a,and the Y axis offset 420, may be used by the processing system 144 tocalculate a zero offset to compensate for offset conditions, and a gainfactor to compensate for slope differences. In the example of FIG. 11a,a zero offset may be the negative of the Y axis offset value 420, andthe gain factor may be 1.

Referring now to FIG. 11b, a graph 430 includes measurement data points410 b, 412 b, 414 b, and 416 b along a measurement correlation curve 406b, corresponding to another set of width and pitch measurements by theSEM system 100 along scan line 378 of line sets 370, 372, 374, and 376in FIG. 10. In this example, a correlation of the measurement curve 406b with theoretical curve 402 shows that there is no Y axis offset, butthat the measurement curve slope 418 b is greater than the slope 404 ofthe theoretical curve 402. The processing system 144 may obtain one ormore calibration factors (e.g., gain factor, zero offset factor) basedon the correlation of the measurement data points 410 b, 412 b, 414 b,and/or 416 b. As discussed supra, the calibration factors may then becorrelated with workpiece feature measurements (either prior orsubsequent) to obtain workpiece feature CDs.

Referring now to FIG. 11c, a graph 440 includes measurement data points410 c, 412 c, 414 c, and 416 c along a measurement correlation curve 406c, corresponding to another set of width and pitch measurements by theSEM system 100 along scan line 378 of line sets 370, 372, 374, and 376in FIG. 10. In graph 440, the slope 418 c of the correlation curve 406 cis less than the slope 404 of the theoretical curve 412. In addition, anon-zero Y axis offset 442 is present. The processing system 144 mayobtain one or more calibration factors (e.g., gain factor, zero offsetfactor) based on the correlation of the measurement data points 410 c,412 c, 414 c, and/or 416 c. The calibration factors may then becorrelated with workpiece feature measurements to obtain workpiecefeature CDs.

The correlations of FIGS. 11a, 11 b, and 11 c illustrate the concept ofcorrelation of dimensional and spatial information (e.g., line width andpitch data) in calibration an SEM. Referring now to FIGS. 12a, 12 b, and12 c, exemplary correlations are graphically illustrated wherein actualpitch information and measured pitch information are correlated. Againreferring to the reference sample 350, pitch measurements 510 a, 512 a,514 a, and 516 a may be taken of the line sets 370, 372, 374, and 376along scan line 378. The measurements 510 a, 512 a, 514 a, and 516 a areused by the processing system 144 to construct a measurement correlationcurve 506 a illustrated in the graph 500 of FIG. 12a. The slope 518 a ofthe correlation curve 506 a in this example is the same as the slope 514of a theoretical curve 502, and a Y axis offset 520 exists. Were thepitch measurements 510 a, 512 a, 514 a, and 516 a the same as the actualpitch (e.g., previously known pitch) values for the line sets 370, 372,374, and 376 of reference sample 350, the curves 506 a and 502 would beidentical. In this example, however, the correlation by the processingsystem 144 indicates that the Y axis offset 520 exists, which mayindicate that the magnification of the SEM system is in need ofadjustment. The system 144 correlates the measurement informationthrough one or more mathematical techniques in order to obtain one ormore calibration factors (e.g., zero offset and gain factors) for use inobtaining workpiece measurement CDs as discussed supra.

Referring also to FIGS. 12b and 12 c, graphs 530 and 540 illustrateother measurements (510 b, 512 b, 514 b, 516 b, and 510 c, 512 c, 514 c,516 c, respectively) of the line sets 370, 372, 374, and 376 ofreference sample 350. Correlation curve 506 b has no Y axis offset, andthe slope 518 b thereof is greater than the slope 504 of the theoreticalcurve 502. Correlation curve 506 c has a Y axis offset 542 associatedtherewith, and the slope 518 c thereof is less than the slope 504. Thesystem 144 correlates the measurements in determining calibrationfactors for the system 100, as discussed supra. The theoretical curve502 in graphs 500, 530, and 540 will generally have a slope 504 equal to1, since the X axis and Y axis both represent pitch.

In similar fashion, measured and actual width information may becorrelated, as illustrated in graphs 600, 630, and 640 of FIGS. 13a, 13b, and 13 c, respectively. The measurements 610, 612, 614, and 616 ofcorrelation curves 606 are of the line sets 370, 372, 374, and 376 ofreference sample 350. The processing system 144 may correlate themeasurements 610, 612, 614, and/or 616 to obtain one or more calibrationfactors for use in obtaining workpiece CDs. The correlation may takeinto account variations in curve slopes 618 from the theoretical value604, as well as Y axis offsets 620, 642, etc., as described with respectto FIGS. 11a, 11 b, 11 c, 12 a, 12 b, and 12 c supra. In addition tocorrelating measurement information to obtain one or more calibrationfactors, the system 144 may provide trending and other information to auser, for example, relating to the degradation of one or more featureson a reference sample. Based on this information, a user may decide todiscard a degraded reference sample and replace it with a new one, etc.

Referring now to FIG. 14a, a graph 700 includes measurements 710 a, 712a, 714 a, and 716 a relating to SEM line width measurements along scanline 378 of line sets 370, 372, 374, and 376, respectively, of thereference sample 350 in FIG. 10. A measured line width curve 706 a isshown with a slope 718 a, along with a theoretical curve 702 with aslope 704. In this example, the measurement 712 a is above thecorrelation curve 706 a. This may indicate that the line features (notnumerically designated) of the line set 372 have degraded, or otherwisehave increased in width. This may also indicate some non-linearity inthe SEM system 100.

The processing system 144 may utilize various curve fitting techniquessuch as, for example, computing the slope of a curve, computing a zerooffset, computing a calibration coefficient, curve fitting, stochastics,neural networks, artificial intelligence, data fusion techniques, and/ortrending to determine the correlation curve 706 a to be used inobtaining calibration factors for the system 100. As an example, thecurve 706 a is illustrated in FIG. 14a as generally linear, with thereading 712 a discounted or discarded. In this regard, the system 144may determine, for example, through trending analysis and the like, thata certain reference sample feature or line set, such as set 372, isunreliable, and may account for this in subsequent correlations. Thecorrelations illustrated in FIGS. 14a, 14 b, and 14 c, show curvefitting wherein the correlation curves 706 a, 706 b, and 706 c,respectively, have been determined with measurement points 712 a, 714 b,and 710 c, respectively, discounted in the correlation. It will beappreciated that other correlations are possible within the scope of theinvention wherein, for example, non-linear correlation curves (notshown) are constructed, or where the offsets 746, 748, and 750 inmeasurements 712 a, 714 b, and 710 c, respectively, are otherwise takeninto account. In this regard, system 144 may correlate the measurementinformation in a variety of ways, selectively taking into account some,none, or all non-linearities, offsets, slopes, etc., in obtaining one ofmore calibration factors for the SEM system 100.

Referring now to FIG. 15, a graph 800 illustrates a correlation ofactual and measured feature width obtained by measurements 810, 812,814, and 816 via SEM scanning of line sets 370, 372, 374, and 376,respectively, of the reference sample 350 in FIG. 10. A correlationcurve 806 has been constructed in accordance with the invention. At alater time, subsequent measurements 860, 862, 864, and 866 are obtainedof the same lines sets 370, 372, 374, and 376. A correlation curve 856is illustrated for this second measurement of the reference features,which may be used by the processing system 144 to obtain one or morecalibration factors (not shown) for the SEM system 100, as discussedsupra. The system 144 may further perform trending analysis of theprevious and subsequent curves 806 and 856, respectively. In thisexample, the system 144 may indicate the change over time to a user, maymake adjustments in the SEM system 100 based thereon, may correlate thistrending information with other reference sample feature measurementsusing, for example, data fusion techniques, and/or may take otherappropriate actions.

Many different correlations are possible, all of which are notillustrated herein. However, it will be appreciated that a wide varietyof correlation techniques are within the scope of the invention,comprising one or more of computing the slope of a curve, computing azero offset, computing a calibration coefficient, curve fitting,stochastics, neural networks, artificial intelligence, data fusiontechniques, and/or trending. In addition, many different correlationsare possible, based on measurements of features on different referencesamples, some of which are illustrated in FIGS. 4-10.

Although the invention has been shown and described with respect to acertain embodiments, it will be appreciated that equivalent alterationsand modifications will occur to others skilled in the art upon thereading and understanding of this specification and the annexeddrawings. In particular regard to the various functions performed by theabove described components (assemblies, devices, circuits, systems,etc.), the terms (including a reference to a “means”) used to describesuch components are intended to correspond, unless otherwise indicated,to any component which performs the specified function of the describedcomponent (i.e., that is functionally equivalent), even though notstructurally equivalent to the disclosed structure, which performs thefunction in the herein illustrated exemplary embodiments of theinvention. In this regard, it will also be recognized that the inventionincludes a system for performing the steps of the various methods of theinvention.

In addition, while a particular feature of the invention may have beendisclosed with respect to only one of several embodiments, such featuremay be combined with one or more other features of the other embodimentsas may be desired and advantageous for any given or particularapplication. Furthermore, to the extent that the terms “includes”,“including”, “has”, “having”, and variants thereof are used in eitherthe detailed description or the claims, these terms are intended to beinclusive in a manner similar to the term “comprising.”

What is claimed is:
 1. A method for calibrating a scanning electronmicroscope, comprising: providing a reference sample having a first linewith a first line width, and a second line with a second line width;measuring the first line width using the scanning electron microscope;measuring the second line width using the scanning electron microscope;and correlating the first line width measurement with the second linewidth measurement to obtain at least one calibration factor.
 2. Themethod of claim 1, further comprising: measuring a workpiece featureusing the scanning electron microscope to obtain a workpiece featuremeasurement; and correlating the workpiece feature measurement with theat least one calibration factor to obtain a workpiece feature criticaldimension (CD).
 3. The method of claim 1, wherein correlating the firstline width measurement with the second line width measurement to obtainat least one calibration factor comprises at least one of computing theslope of a curve, computing a calibration coefficient, curve fitting,stochastics, neural networks, artificial intelligence, data fusiontechniques, and trending.
 4. A method for calibrating a scanningelectron microscope, comprising: providing a reference sample having afirst line set with generally parallel lines of a first pitch, and asecond line set with generally parallel lines of a second pitch;measuring the first pitch using the scanning electron microscope;measuring the second pitch using the scanning electron microscope; andcorrelating the first pitch measurement with the second pitchmeasurement to obtain at least one calibration factor.
 5. The method ofclaim 4, further comprising: measuring a workpiece feature using thescanning electron microscope to obtain a workpiece feature measurement;and correlating the workpiece feature measurement with the at least onecalibration factor to obtain a workpiece feature critical dimension(CD).
 6. The method of claim 5, wherein correlating the workpiecefeature measurement with the at least one calibration factor to obtain aworkpiece feature critical dimension (CD) comprises at least one ofcomputing the slope of a curve, computing a calibration coefficient,curve fitting, stochastics, neural networks, artificial intelligence,data fusion techniques, and trending.
 7. The method of claim 6, whereincorrelating the first pitch measurement with the second pitchmeasurement to obtain at least one calibration factor comprises at leastone of computing the slope of a curve, computing a calibrationcoefficient, curve fitting, stochastics, neural networks, artificialintelligence, data fusion techniques, and trending.
 8. The method ofclaim 4, wherein correlating the first pitch measurement with the secondpitch measurement to obtain at least one calibration factor comprises atleast one of computing the slope of a curve, computing a calibrationcoefficient, curve fitting, stochastics, neural networks, artificialintelligence, data fusion techniques, and trending.
 9. A method forcalibrating a scanning electron microscope, comprising: providing areference sample having a first line set with generally parallel linesof a first line width and a first pitch, and a second line set withgenerally parallel lines of a second line width and a second pitch;measuring at least one of the first line width and the first pitch usingthe scanning electron microscope; measuring at least one of the secondline width and the second pitch using the scanning electron microscope;and correlating at least one of the first line width measurement and thefirst pitch measurement with at least one of the second line widthmeasurement and the second pitch measurement to obtain at least onecalibration factor.
 10. The method of claim 9, wherein correlating atleast one of the first line width measurement and the first pitchmeasurement with at least one of the second line width measurement andthe second pitch measurement to obtain at least one calibration factorcomprises at least one of computing the slope of a curve, computing acalibration coefficient, curve fitting, stochastics, neural networks,artificial intelligence, data fusion techniques, and trending.
 11. Themethod of claim 9, further comprising: measuring a workpiece featureusing the scanning electron microscope to obtain a workpiece featuremeasurement; and correlating the workpiece feature measurement with theat least one calibration factor to obtain a workpiece feature criticaldimension (CD).
 12. The method of claim 11, wherein correlating at leastone of the first line width measurement and the first pitch measurementwith at least one of the second line width measurement and the secondpitch measurement to obtain at least one calibration factor comprisescomputing a slope of a curve connecting at least one of the first linewidth measurement and the first pitch measurement with at least one ofthe second line width measurement and the second pitch measurement toobtain a calibration zero factor and a calibration span factor.
 13. Themethod of claim 9, wherein correlating at least one of the first linewidth measurement and the first pitch measurement with at least one ofthe second line width measurement and the second pitch measurement toobtain at least one calibration factor comprises computing a slope of acurve connecting at least one of the first line width measurement andthe first pitch measurement with at least one of the second line widthmeasurement and the second pitch measurement to obtain a calibrationzero factor and a calibration span factor.
 14. A system for calibratinga scanning electron microscope, comprising: a reference sample having afirst line set with generally parallel lines of a first line width and afirst pitch, and a second line set with generally parallel lines of asecond line width and a second pitch; a scanning electron microscope formeasuring at least one of the first line width and the first pitch, andat least one of the second line width and the second pitch; and aprocessor for correlating at least one of the first line widthmeasurement and the first pitch measurement with at least one of thesecond line width measurement and the second pitch measurement to obtainat least one calibration factor.
 15. The system of claim 14, wherein theprocessor further correlates a workpiece feature measurement with the atleast one calibration factor in order to obtain a workpiece featurecritical dimension (CD).
 16. The system of claim 14, wherein theprocessor correlates the at least one of the first line widthmeasurement and the first pitch measurement with at least one of thesecond line width measurement and the second pitch measurement to obtainat least one calibration factor via at least one of computing the slopeof a curve, computing a calibration coefficient, curve fitting,stochastics, neural networks, artificial intelligence, data fusiontechniques, and trending.
 17. A system for calibrating a scanningelectron microscope, comprising: a reference sample having a first lineset with generally parallel lines of a first line width and a firstpitch, and a second line set with generally parallel lines of a secondline width and a second pitch; means for measuring at least one of thefirst line width and the first pitch; means for measuring at least oneof the second line width and the second pitch; and means for correlatingat least one of the first line width measurement and the first pitchmeasurement with at lest one of the second line width measurement andthe second pitch measurement to obtain at least one calibration factor.18. The system of claim 17, further comprising means for correlating aworkpiece feature measurement with the at least one calibration factorto obtain a workpiece feature critical dimension (CD).
 19. The system ofclaim 18, wherein the means for correlating a workpiece featuremeasurement with the at least one calibration factor to obtain aworkpiece feature critical dimension (CD) comprises at least one ofcomputing the slope of a curve, computing a calibration coefficient,curve fitting, stochastics, neural networks, artificial intelligence,data fusion techniques, and trending.
 20. The system of claim 17,wherein the means for correlating at least one of the first line widthmeasurement and the first pitch measurement with at least one of thesecond line width measurement and the second pitch measurement to obtainat least one calibration factor comprises at least one of computing theslope of a curve, computing a calibration coefficient, curve fitting,stochastics, neural networks, artificial intelligence, data fusiontechniques, and trending.